1. Field of the Invention
The present invention relates to a solid electrolytic fuel cell. More particularly, the invention relates to a solid electrolytic fuel cells having an oxygen electrode layer that is provided on a solid electrolytic layer via a reaction-preventing layer and to a fuel cell assembly.
2. Description of the Related Art
In recent years, a variety of fuel cell assemblies have been proposed as the energy of the next generation accommodating stacks of fuel cells in a container. As the fuel cells of these kinds, there have been known those of the solid high molecular type, phosphoric acid type, molten carbonate type and solid electrolytic type. Among them, despite of its operation temperature of as high as 800 to 1000° C., the fuel cell of the solid electrolytic type features a high generation efficiency offering such an advantage as utilizing waste heat, and its study and development have been forwarded.
The solid electrolytic fuel cell has a basic structure in which a fuel electrode is provided on one surface of the solid electrolytic layer and an oxygen electrode (air electrode) is provided on the other surface thereof. In this solid electrolytic fuel cell, in general, the oxygen ion conductivity of the solid electrolyte starts increasing at about 600° C., and a gas containing oxygen is supplied to the oxygen electrode side and a gas containing hydrogen is supplied to the fuel electrode side at the temperature of not lower than 600° C., so that a potential difference generates across the oxygen electrode and the fuel electrode based upon a difference in the oxygen concentration between the two electrodes.
Oxygen ions that migrate from the oxygen electrode to the fuel electrode through the solid electrolyte bond to hydrogen ions in the fuel electrode to form water. Here, electrons migrate simultaneously. In the fuel cell, therefore, the gas containing oxygen and the gas containing hydrogen are supplied to continuously trigger the above reaction to generate electricity. That is, electricity is generated by the electrode reactions on the oxygen electrode and on the fuel electrode as expressed by the following formulas:Oxygen electrode: 1/2O2+2e−→O2−(solid electrolyte)Fuel electrode: O2−(solid electrolyte)+H2→H2O+2e−
In a solid electrolytic fuel cell that is usually used, the above cell structure is formed on a porous electrically conducting support member having gas passages therein, a fuel gas (hydrogen gas) is flown into the gas passages in the electrically conducting support member, hydrogen is supplied onto the surface of the fuel electrode via the electrically conducting support member and, at the same time, an oxygen-containing gas such as the air is flown onto the outer surface of the oxygen electrode thereby to supply oxygen onto the surface of the oxygen electrode, so that the above-mentioned electrode reactions take place on the electrodes, and an electric current that is generated is taken out through an interconnector provided on the electrically conducting support member. A plurality of fuel cells of this structure are connected in series by using collector members to form a cell stack. A plurality of these cell stacks are contained in a suitable container and are connected together by using conducting members so as to be used as a fuel cell assembly.
In the above solid electrolytic fuel cell, an La—Sr—Co perovskite composite oxide has been widely used as a material for forming the oxygen electrode owing to its surface diffusing function and volume diffusing function for oxygen ions and excellent operation at low temperatures (see Japanese Unexamined Patent Publication (Kokai) No. 08-130018).
In the conventional known solid electrolytic fuel cell, however, elements are diffused from the oxygen electrode to the solid electrolytic layer due to the heating at the time of generating electricity or at the time of production, arousing a problem in that an insulating layer is formed on the interface between the oxygen electrode and the solid electrolytic layer due to the diffusion of elements. To prevent the diffusion of elements, there has been proposed to provide a reaction-preventing layer comprising an oxide (e.g., Ce oxide) having ion conductivity and electron conductivity between the solid electrolytic layer and the oxygen electrode (see Japanese Unexamined Patent Publication (Kokai) No. 2002-15754).
Even when the above reaction-preventing layer is provided, however, performance of the fuel cell drops after the electricity is generated for extended periods of time due to a high interfacial resistance between the oxygen electrode and the reaction-preventing layer. As means for solving the above problem, Japanese Unexamined Patent Publication (Kokai) No. 2002-289248 proposes a technology in which the oxygen electrode is formed in a two-layer structure including a lower layer and an upper layer, the lower layer (i.e., layer on the side of the reaction-preventing layer) being a dense layer of particles of small sizes to decrease the interfacial resistance to the reaction-preventing layer, and the upper layer being a porous layer of particles of large sizes to increase the three-phase interfaces. However, this means still involves a problem of a weak mechanical junction strength between the lower layer of the oxygen electrode and the reaction-preventing layer, permitting the oxygen electrode layer to be peeled off the reaction-preventing layer after the electric generation of extended periods of time. Further, the solid electrolytic fuel cell provided with the reaction-preventing layer is still accompanied by a common problem in that the function of the oxygen electrode layer is not exhibited to a sufficient degree, and that the output is lower than that of the solid electrolytic fuel cell without the reaction-preventing layer.